Cyclopropane-cyclopentane ring

At one time all cycloalkanes were believed to be planar It was expected that cyclopentane would be the least strained cycloalkane because the angles of a regular pentagon (108°) are closest to the tetrahedral angle of 109 5° Heats of combustion established that this is not so With the exception of cyclopropane the rings of all cycloalkanes are nonplanar... 

In addition to unsaturated fatty acids, several other modified fatty acids are found in nature. Microorganisms, for example, often contain branched-chain fatty acids, such as tuberculostearic acid (Figure 8.2). When these fatty acids are incorporated in membranes, the methyl group constitutes a local structural perturbation in a manner similar to the double bonds in unsaturated fatty acids (see Chapter 9). Some bacteria also synthesize fatty acids containing cyclic structures such as cyclopropane, cyclopropene, and even cyclopentane rings. 

Cycloalkanes. Cycloalkanes are conformationally restricted alkanes. Three rings are employed in drug design cyclopropane, cyclopentane, cyclohexane (the latter two are... 

Recent advances in Cp-based catalyst technology made it possible to produce unique microstructure polymers from ethylene and BD. Longo and co-workers have reported in a series of publications the unprecedented cyclo-co-polymerization of ethylene and BD using a sterically encumbered isospecific metallocene F13-8 with MAO, which affords 1,2-cyclopropane rings together with 1,2-cyclopentane rings in the polymer chain, both with high trans-... 

Lithium tributylmagnesate induced iodine-magnesium exchange reaction of 5-alkoxy-3-iodomethyl-l-oxacyclopentanes (Scheme 16). A following intramolecular nucleophilic substitution led to construction of a cyclopropane with concomitant opening of the oxa-cyclopentane ring. 

Finally, we describe the two syntheses of [3]peristylane 403) reported by Garratt and White.369 This hydrocarbon is constructed of three cyclopentane rings which are mutually fused in a manner which also generates a cyclopropane ring. Subsequent to the conversion of norbomadiene to endo-3-carboxybicyclo[3.2.1 ]oct-6-ene 401) by an established route,370 this acid was reduced and reoxidized to the aldehyde level (Scheme 65). Pyrolysis of the sodium salt of the corresponding tosyl-... 

The very high nucleophilicity of a-heterosubstituted a-lithiocyclopropanes and of a-lithioalkyl selenides, even those bearing two alkyl groups on the, carbanionic center, permits the stepwise construction of a cyclopentane ring possessing several quaternary centers in vicinal positions with respect to one another by two successive ring expansion reactions from a suitably functionalyzed cyclopropane 135,224) (Schemes 98, 101). This feature has been used by Fitjer2240 for the synthesis of permethyl cyclobutanone, permethyl cyclopentanone, and even permethyl cyclohexanone, as well as for the preparation of previously unknown permethylcyclohexane. 

The marine alga Laurencia hybrida produces a compound that has been isolated and characterized as hybridalactone (73). Structurally, this compound contains a 13-membered lactone ring fused to a cyclopentane ring. The macrocyclic ring, in addition to containing two (Z)-olefins, has a substituted cyclopropane ring appended ct to the lactone oxygen. 

An analysis of multipath transmission of spin-spin coupling in cyclic compounds has been made by Wu and Cremer with the help of partially spin-polarized orbital contributions. The calculations have been performed for cyclopropane, cyclopentane, bicydo[1.1.1]pentane and tetrahydrofuran. The authors have shown that the measured and calculated Vcc couplings of cyclopentane and tetrahydrofuran are averages over the pseudorotational motion of these ring molecules where each individual coupling of a conformation passed in the pseudorotation is the sum of different path contributions. 

In situ formed iminiums were also able to react with alkyltins in an intramolecular fashion, leading to the formation of cyclopropane [265] or cyclopentane rings (Scheme 6.30). Similarly, the intramolecular reaction of y-alkoxystarmane with hydrazones, activated by a Lewis acid, was used to prepare 5- or 6-membered /3-amino cyclic ethers, for which the trans preference for the cyclization was consistent with an acyclic transition state [266]. 

Some straightforward, efficient cyclopentanellation procedures were developed recently. Addition of a malonic ester anion to a cyclopropane-1,1-dicarboxylic ester followed by a Dieckmann condensation (S. Danishefsky, 1974) or addition of iJ-ketoester anions to a (l-phenylthiocyclopropyl)phosphonium cation followed by intramolecular Wittig reaction (J.P, Marino. 1975) produced cyclopentanones. Another procedure starts with a (2 + 21-cycloaddition of dichloroketene to alkenes followed by regioselective ring expansion with diazomethane. The resulting 2,2-dichlorocyclopentanones can be converted to a large variety of cyclopentane derivatives (A.E. Greene. 1979 J.-P. Deprds, 1980). 

Physical properties of cycloalkanes [49, p. 284 50, p. 31] show reasonably gradual changes, but unlike most homologous series, different members exhibit different degrees of chemical reactivity. For example, cyclohexane is the least reactive member in this family, whereas both cyclopropane and cyclobutane are more reactive than cyclopentane. Thus, hydrocarbons containing cyclopentane and cyclohexane rings are quite abundant in nature. 

The data in Figure 4.3 show that Baeyer s theory is only partially correct. Cyclopropane and cyclobutane are indeed strained, just as predicted, but cyclopentane is more strained than predicted, and cyclohexane is strain-free. Cycloalkanes of intermediate size have only modest strain, and rings of 14 carbons or more are strain-free. Why is Baeyer s theory wrong ... 

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